This invention relates to microelectromechanical mirrors and mirror arrays, and a method for manufacturing the same.
As the internet has grown, so too has the strain on the telecommunications infrastructure. As more and more information is transmitted across the Internet, and the demand for information rich content like streaming video has grown, telecommunication providers have struggled to provide the necessary bandwidths and data rates necessary to carry the requisite data. To that end, telecommunications providers have looked to carrying more and more data on fiber optic networks, and to extending the reach of their fiber optic networks beyond the network backbone.
One limitation of fiber optic networks as currently implemented is their inability to directly switch optically encoded packets of data from a fiber on a source network or network node to a fiber on a destination network or network node. Instead, the optically encoded data is dropped from the source network fiber, converted to electrically encoded data, switched to the destination network using conventional electronic switches, converted back into optically encoded data, and injected into the destination network fiber.
Micromachined mirror arrays offer the ability to directly switch optically encoded data in devices known as all-optical cross connect switches from a source fiber on a source network to a destination fiber on a destination network without having to convert the data from optical to electronic and back again. For such mirror arrays to be commercially useful, they must be able to cross connect approximately 1000 input fibers with an equal number of output fibers in a compact volume. This can be achieved with mirrors that can be densely packed together and that are rotatable by relatively large angles (xcx9c5xc2x0) in an arbitrary angular direction.
Recent developments in the field of microelectomechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches. MEMS-based mirrors and mirror arrays can be inexpensively designed and produced using conventional tools developed for the design and production of integrated circuits (IC""s). Such tools include computer aided design (CAD), photolithography, bulk and surface micromachining, wet and dry isotropic and anisotropic etching, and batch processing. In addition, deep reactive ion etching methods (DRIE) allow silicon devices to be produced having high aspect ratios (xcx9c20:1) that rival those that can be achieved using the prohibitively expensive lithography, electroplating and molding process (LIGA) which requires access to a synchrotron radiation source. (LIGA is an acronym for the German lithographie, galvanoformung und abformung).
A number of microelectromechanical mirror arrays have already been built using MEMS production processes and techniques. These arrays have designs that fall into approximately three design categories, each of which suffers from one or more limitations that make them sub-optimal for use in an all-optical cross connect switch.
The first and simplest design is illustrated by U.S. Pat. No. 5,960,132 to Lin. In this design, a reflective panel is hinged to a reference base and is electrostatically rotated about the hinge. Since the panel""s freedom of motion is limited to rotation about the hinge, light incident on the panel cannot be reflected in an arbitrary angular direction (dxcex8, dxcfx86) but only along an arc defined by a single angle, i.e., dxcex8 or dxcfx86. As a result, light incident from a source fiber cannot be directed to an arbitrary output fiber but only to those output fibers located along the defined arc. Consequently, Lin""s system requires large and costly system redundancies to connect a plurality of input fibers to a plurality of output fibers. These redundancies can be in either the number of output fibers or in the number of mirrors. In Lin, the redundancy is in the number of mirrors, where N2 mirrors are used to connect N input fibers to N output fibers. An optimal system would only require N mirrors to make the N input to N output possible fiber interconnections.
A more sophisticated design is illustrated in U.S. Pat. No. 6,044,705 to Neukermans et al which is hereby incorporated by reference. In Neukermans, a gimbal is mounted on a first hinge connected to a reference surface, while a mirror is mounted on a second hinge connected to the gimbal. The first and second hinges are orthogonal to each other and allow the mirror to be rotated in an arbitrarily angular direction (dxcex8, dxcfx86). The gimbal is electrostatically rotated about the first hinge by applying a potential between it and electrodes located on the reference surface. The mirror is electromagnetically rotated about the second hinge by injecting a current in a conductive coil wrapped around the mirror perimeter. The current flow through the coil generates a small magnetic moment which couples to a permanent magnetic field established across the plane of the mirror (e.g. with bar magnets), and causes the mirror to rotate. While Neukermans use of a gimbal thus allows the mirrored surface to rotate in an arbitrary angular direction, it also makes the system more mechanically and electrically complex than it needs to be. The mechanical complexity increases the sensitivity of the system to mechanical vibrations, while the electrical complexity increases the intricacy of the electrostatic and electromagnetic actuators. Both complexities increase the cost of producing the system. Additionally, Neukermans electromagnetic actuator coil occupies a large amount of the surface of the device, thus reducing the mirrored surface area and the mirror density.
A third mirror design is illustrated in U.S. Pat. No. 6,040,953 to Michalicek. In Michalicek, a mirror is mounted on a central post anchored to a locking pin joint that is carved into a reference surface. The post can be electrostatically actuated to freely rotate about the pin joint in an arbitrary direction. However, because the post is not mechanically attached to the pin joint with flexures, it can only be stably rotated in directions where the mirrored surface can be supported by a landing pad provided for that purpose. The mirror can therefore only be rotated and held in a fixed number of stable positions. In Michalicek""s preferred embodiment, the mirror can only be rotated to and held in two stable positions.
The invention discloses a method for manufacturing a freely movable plate and a microelectromechanical mirror and mirror array utilizing the freely movable plate. The freely movable plate, and microelectromechanical mirror and mirror array can be fabricated from a plurality of silicon substrates using standard IC processing steps such as wet and dry chemical etching, photolithography, bulk and surface silicon micromachining, and deep reactive ion etching.
In one aspect, the invention discloses a method for manufacturing a freely movable plate from an actuation layer wafer and a reference layer wafer. The actuation layer wafer can be a single crystal silicon wafer such as a silicon-on-oxide (SOI) wafer. A photoresist layer can be deposited onto the actuation layer wafer and patterned with a mask for one or more actuators that may be electrostatically, electromagnetically, piezoelectrically or thermally actuated. The actuation layer mask can include masking for a support frame, a plurality of actuators and actuator flexures, a freely movable plate, and a plurality of plate flexures. The photoresist layer can be patterned with alignment marks to align the actuation layer wafer with a mirror layer wafer in a microelectromechanical mirror. Portions of the actuation layer wafer exposed by the actuator mask can be etched away using a high aspect ratio etch such as a DRIE etch. The remaining photoresist can be stripped away.
The actuation layer can be actuated by actuation means deposited onto, etched into, or etched from a reference layer wafer. A conductive layer including control electrodes and their associated traces can be deposited onto the surface of the reference wafer layer. In one embodiment, the control electrodes are deposited by growing a thermal oxide layer on the reference layer wafer; depositing a conductive layer over the oxide layer; depositing a photoresist layer over the conductive layer; patterning the photoresist layer with a mask for control electrodes and their associated traces; etching the control electrodes and their associated traces from the conductive layer; and stripping away the remaining photoresist.
An optional mechanical stopping layer can be deposited and patterned onto the top surface of the reference layer wafer to electrically isolate the actuation layer structures from the reference layer structures. The mechanical stopping layer can be made from a material such as a polyimide that can provide a small leakage path to ground. The stopping layer can electrically isolate the actuation and reference layer structures, and dissipate any charge buildup that might occur between them to prevent long term voltage drifts between the structures. In one embodiment, the stopping layer is deposited as a polyimide layer that is patterned and etched to produce an array of polyimide dots or a sequence of polyimide stripes on the surface of the reference layer wafer.
A separation layer can be deposited onto the top surface of the reference layer wafer to hold the actuation layer structures above the reference layer structures. In one embodiment, the separation layer is made from a polyimide layer, however, other materials such as low temperature solders may be used. A plurality of standoff posts can be etched into the separation layer to support the actuation layer wafer above the reference layer wafer. The reference and actuation layer wafers can be bonded together using a low temperature bonding technique after aligning the reference layer standoff posts with the actuation layer support frame.
In another aspect, the invention discloses a method for manufacturing a microelectromechanical mirror utilizing the freely movable plate. The mirror can be made from reference and actuation layer wafers as previously described, and from a mirror layer wafer. The mirror layer wafer can be bonded to the actuation layer wafer before the actuation layer wafer is bonded to the reference layer wafer. The mirror layer wafer can be a single crystal silicon wafer. A mirror support post and mirrored surface can be etched from the wafer using a high aspect ratio etch. The mirror support post can be etched from the mirror layer wafer by depositing a hard mask such as an Aluminum mask onto the mirror layer wafer; depositing photoresist over the hard mask; transferring a mirror support post mask to the photoresist, etching away portions of the hard mask exposed by the mirror support post mask; stripping away the photoresist; etching the mirror support post from the mirror layer wafer using a high aspect ratio etch; and stripping the hard mask from the mirror layer wafer. Alignment bores can be etched into the mirror layer wafer to facilitate aligning the mirror layer wafer with the actuation layer wafer.
The mirror layer wafer can then be fusion bonded to the actuation layer wafer such that the mirror support post of the mirror layer wafer is bonded to the freely movable plate of the actuation layer wafer. The mirror and actuation layer wafers can be thoroughly cleaned before they are bonded together using a commercially available wafer cleaning process such as the RCA process. The cleaned wafers can be aligned along their respectively etched alignment bores before being fusion bonded together. Once bonded to the mirror layer wafer, the bulk of the actuation layer wafer can be ground away, while the remainder can be slowly etched away. The combined mirror and actuation layer wafers can then be bonded to the reference layer wafer as previously disclosed.
A reflective layer such as a 1000 xc3x85 thick gold layer can be deposited onto the top surface of the mirror layer wafer. A photoresist can be deposited over the reflective layer and patterned with a mirror mask. Portions of the reflective layer and of the mirror layer wafer exposed by the mirror mask can be etched away to free the mirrored surface using a high aspect ratio etch. The remaining photoresist can be stripped away to the complete microelectromechanical mirror structure.
In another aspect of the invention, the disclosed processes for making an individual microelectromechanical mirror can be readily adapted to make a plurality of mirrors in a mirror array by regularly repeating the process masks needed to make a single mirror. The process masks can be repeated to produce a mirror array of arbitrary geometry, and in one embodiment a 30xc3x9740 mirror array is made for use in an all optical cross connect switch.
The details of various embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description, drawings and claims.